(1) Field of the Invention:
The present invention relates to high strength silicon nitride sintered members. More particularly, the invention relates to silicon nitride sintered members which have excellent oxidation resistance and which can be used as high temperature structural members.
(2) Related Art Statement:
Since silicon nitride sintered bodies have high strength at high temperatures and excellent thermal shock resistance, they are used as gas turbine members or high temperature structural members while their fired surfaces or worked surfaces are exposed. When the silicon nitride sintered bodies which constitute such members by fired surfaces or worked surfaces thereof are used at high temperatures, the strength of the members lowers due to oxidation of the surface of the bodies. Thus, their use life is conspicuously limited when they are used as gas turbine members or high temperature structural members. In view of this, for example, Japanese patent publication No. 61-40,630 or Japanese patent application Laid-open No. 60-161,383 discloses improvement of oxidation resistance of silicon nitride sintered bodies by covering their surfaces with a silicon nitride coating layer of excellent oxidation resistance through a CVD process (chemical vapor disposition process).
However, when even such silicon nitride sintered bodies covered with the silicon nitride by the CVD process are used at high temperatures, there is a problem in that the silicon nitride coating layer reacts with an intergranular phase of the silicon nitride sintered body to cause peeling or breakage of the coating layer so that an oxidation resistance-imparting effect of the coating layer diminishes.